1. Field of the Invention
The present invention relates to a semiconductor sensor for detecting physical quantities such as pressure, acceleration, mechanical vibration, etc.
2. Description of the Relevant Art
There is known a semiconductor sensor which comprises a field-effect transistor on a piezoelectric semiconductor substrate. While a drain current is flowing with a gate voltage applied to the gate of the field-effect transistor, an external force applied to the field-effect transistor can be detected on the basis of a change in the drain current or a drain voltage.
Since the output signal indicative of the applied external force is higher as the drain current is larger, it has been customary to select a gate bias voltage for higher external force sensitivity such that the drain current will be of a large value when no external force is applied.
However, it has experimentally found that the larger drain current for the higher output signal results in a reduction in the accuracy of the output signal. This is because as the drain current increases, the noise current generated by the field-effect transistor itself increases, thereby degrading the ratio of the output signal to the noise (signal-to-noise ratio).